During the last five years, transgenic and knock-out animals as well as fetal thymic organ culture models have catalyzed remarkable progress in the delineation of the molecular events of early murine T lymphopoiesis. Knowledge of parallel events in human thymopoiesis has understandably lagged far behind. This is in part due to a lack of adequate culture models for human T cell differentiation. While some features of T lymphopoiesis are shared between men and mice, there are also significant differences. For example, the phenotypic properties of the various developmental stages in men and mice are different and the TCRbeta chain is the first to rearrange in human thymic ontogeny, while gammadelta development precedes that of alphabeta T cells in mice. While the details of the alphabeta versus gammadelta lineage decisions are not completely understood in mice, information in humans is even more rudimentary. For the above reasons, it may not be appropriate to draw conclusions about major event in human thymopoiesis from information obtained from the study of mice. Accordingly, and in keeping with the Program Project theme of human lymphocyte development, we will undertake a detailed analysis of the molecular events in early human thymopoiesis. The proposed research is divided into three Specific Aims: I. To resolve steps in human thymocyte differentiation. II. To determine whether alphabeta and gammadelta T cells are derived from a common precursor or represent two separate lineages. III. To develop a cell culture system for the efficient production of human thymocytes at all stages of differentiation. A more complete understanding of how T cells are made in man will benefit the understanding and treatment of immunodeficiency diseases, T cell malignancies, and autoimmune diseases.